1. Technical Field
The present invention relates to a self-excited switching power supply circuit that performs continuous oscillating operation by feeding a voltage appearing across a feedback winding of a transformer back positively as a driving signal to a control terminal of a switching element, and more specifically, to a self-excited switching power supply circuit that enhances noise terminal characteristics of an output.
2. Description of the Related Art
A switching power supply circuit functioning as a stabilizing power supply is used in a battery charger, an AC adapter and others. This switching power supply circuit performs constant voltage control to stabilize an output voltage at a predetermined set voltage irrespective of the magnitudes of such loads connected to the output side (see paragraphs 0033 to 0066 of the specification, and FIG. 1 of Japanese Patent No. 3691498 for example).
A self-excited switching power supply circuit 100 that performs the aforementioned conventional constant voltage control is described by using FIGS. 5 and 6. Reference numeral 1 is an unstable DC power supply having a fear of voltage fluctuation. Reference numerals 1a and 1b are a high-voltage side terminal and a low-voltage side terminal respectively of the DC power supply 1. Further, reference numerals 2a and 2c are a primary winding and a secondary output winding of a transformer 2, respectively, and reference numerals 2b and 2d are first and second feedback windings, respectively, provided on the primary side of the transformer 2. The first feedback winding 2b is wound in the same direction as the primary winding 2a, and the second feedback winding 2d is wound in the opposite direction to the primary winding 2a. 
Reference numeral 3 shows an oscillation field effect transistor (hereinafter abbreviated as FET). Reference numeral 21 shows a start-up resistor used to apply a forward bias (in other words, a gate voltage being the same as or higher than a threshold voltage VTH) to the gate of the FET 3 when the circuit is started. An electric resistor 25 connected in series to the start-up resistor 21 has a resistance value smaller than that of the start-up resistor 21. Thus, when the voltage of the DC power supply 1 is divided at a junction J1 between the start-up resistor 21 and the electric resistor 25 and if a low DC voltage is output, the circuit is not started.
Reference numeral 12 is an ON control capacitor which forms an ON driving circuit together with a feedback resistor 23, and which is connected in series to the feedback resistor 23 between the first feedback winding 2b and the gate of the FET 3. Reference numeral 24 is an electric resistor provided to prevent excessive input to the gate of the FET 3. Reference numeral 5 is an OFF control transistor having a collector connected to the gate of the FET 3, and an emitter connected to the low-voltage side terminal 1b. 
One side of the second feedback winding 2d is connected to the low-voltage side terminal 1b of the DC power supply 1 through a rectifying diode 54 and an output control capacitor 55 connected in series, and the opposite side of the second feedback winding 2d is directly connected the low-voltage side terminal 1b of the DC power supply 1, thereby forming a closed loop. The rectifying diode 54 is arranged such that the forward direction thereof agrees with a direction in which the output control capacitor 55 is charged with a flyback voltage generated in the second feedback winding 2d. 
A junction J2 between the rectifying diode 54 and the output control capacitor 55 is connected through a photocoupler light receiving element 39 to a base J3 of the OFF control transistor 5. An OFF control capacitor 53 is connected between the base J3 and the low-voltage side terminal 1b. 
The base J3 of the OFF control transistor 5 is also connected through a charging and discharging resistor 50 to a junction J4 between the FET 3 and a shunt resistor 51. The OFF control capacitor 53 is charged with a voltage generated in the shunt resistor 51 as a result of flow of a primary winding current through the shunt resistor 51. If the base voltage of the base J3 reaches the operating voltage of the OFF control transistor 5, continuity is formed between the collector and the emitter of the OFF control transistor 5.
The photocoupler light receiving element 39 is put into operation by being optically coupled to a photocoupler light emitting element 35 belonging to the secondary side of the transformer 2. If receiving light from the photocoupler light emitting element 35, the photocoupler light receiving element 39 causes a discharging current from the output control capacitor 55 to flow from the junction J2 to the base J3 in response to the amount of the received light.
Reference numerals 4 and 13 shown on the same side as the secondary output winding 2c are a rectifying diode and a smoothing capacitor, respectively, forming a rectifying and smoothing circuit. The rectifying diode 4 and the smoothing capacitor 13 rectify and smooth the output of the secondary output winding 2c, and give the resultant output between a high-voltage side output line 20a and a low-voltage side output line 20b. 
Voltage dividing resistors 30 and 31 are connected in series between the high-voltage side output line 20a and the low-voltage side output line 20b. A voltage dividing point 32 of the voltage dividing resistors 30 and 31 is connected to the inverting input terminal of an error amplifier 33. Accordingly, an output detecting voltage obtained by dividing an output voltage is applied to the inverting input terminal. A reference power supply 34 is connected between the non-inverting input terminal of the error amplifier 33 and the low-voltage side output line 20b. Accordingly, a reference voltage to be compared with the output detecting voltage is applied to the non-inverting input terminal. The reference voltage is a voltage obtained by dividing a predetermined set voltage at the voltage dividing resistors 30 and 31, the set voltage being used for the constant voltage control between the high-voltage side output line 20a and the low-voltage side output line 20b. Accordingly, the output of the error amplifier 33 indicates a difference of an output voltage from the set voltage.
The photocoupler light emitting element 35 is connected to the output side of the error amplifier 33. The photocoupler light emitting element 35 is connected through an electric resistor 36 to the high-voltage side output line 20a, and flashes on and off according to the output value of the error amplifier 33. As a result, the photocoupler light emitting element 35 emits light of an amount corresponding to the aforementioned difference between the voltages, and the photocoupler light receiving element 39 belonging to the primary side and optically coupled to the photocoupler light emitting element 35 causes a current responsive to the difference between the voltages to flow from the junction J2 to the base J3.
In the self-excited switching power supply circuit 100 of the aforementioned structure, if a DC voltage is applied first between the high-voltage side terminal 1a and the low-voltage side terminal 1b of the DC power supply 1, the ON control capacitor 12 (in FIG. 5, the lower electrode is a positive electrode and the upper electrode is a negative electrode) is charged through the start-up resistor 21 to increase the charging voltage of the ON control capacitor 12 gradually. If the charging voltage of the ON control capacitor 12 reaches the threshold voltage VTH, a forward bias voltage is applied to the gate of the FET 3, thereby turning the FET 3 on (forming continuity between the drain and the source).
When the FET 3 is turned on and an exciting current starts to flow from the DC power supply 1 to the primary winding 2a connected in series to the FET 3, induced electromotive force is generated in each winding of the transformer 2 to store exciting energy in the transformer 2. A voltage generated at this time at a junction J4 between the shunt resistor 51 and the FET 3 as a result of flow of a current in the primary winding 2a is supplied through the charging and discharging resistor 50 to the OFF control capacitor 53 for charging. The current flowing in the primary winding 2a increases in proportion to the time elapsed after the FET 3 is turned on. Accordingly, if the charging voltage of the OFF control capacitor 53 reaches the operating voltage of the OFF control transistor 5, continuity is formed between the collector and the emitter of the OFF control transistor 5. This brings the gate of the FET 3 into a state where the gate of the FET 3 is substantially shorted by the OFF control transistor 5, thereby turning the FET 3 off (hereinbelow, a period from when the FET 3 is turned on and until when the FET 3 is turned off is called an ON operation period, and a period from when the FET 3 is turned off and until when the FET 3 is turned on next time is called an OFF operation period).
When the FET 3 is turned off and a current flowing in the transformer 2 is interrupted substantially, a voltage what is called a flyback voltage (induced counter-electromotive force) is generated in each winding. A flyback voltage generated at this time in the secondary output winding 2c is rectified and smoothed by the rectifying and smoothing circuit composed of the rectifying diode 4 and the capacitor 13, and is output as electric power to be supplied to a load connected between the output lines 20a and 20b. 
Further, a load connected to the output side makes a flyback voltage generated in the first feedback winding 2b proportionate to the flyback voltage generated in the secondary output winding 2c. The flyback voltage generated in the feedback winding 2b is supplied to the ON control capacitor 12 for charging (in FIG. 5, the lower electrode is a positive electrode and the upper electrode is a negative electrode).
During the OFF operation period, a discharging current flows from the OFF control capacitor 53 into the charging and discharging resistor 50 and the shunt resistor 51 to reduce the charging voltage of the OFF control capacitor 53, namely the base voltage of the OFF control transistor 5 to the operating voltage or lower. Further, an equivalent diode is formed between the base and the collector of the OFF control transistor 5. The ON control capacitor 12 is charged with the flyback voltage generated in the first feedback winding 2b through a charging path formed by the shunt resistor 51, the charging and discharging resistor 50, the base and the collector of the OFF control transistor 5, and the feedback resistor 23.
If emission of electric energy from in the secondary output winding 2c is finished, the flyback voltage of the feedback winding 2b functioning as a negative bias for the gate of the FET 3 drops. Then, a charging voltage stored in the ON control capacitor 12 makes the gate voltage of the FET 3 exceed the threshold voltage VTH to turn the FET 3 on again. A series of the aforementioned oscillating operation is repeated.
Energy stored in the transformer 2 in one oscillation cycle is substantially proportionate to the square of energy stored during the ON operation period of the FET 3. The photocoupler light emitting element 35 does not emit light if the output voltage of the secondary side does not reach the set voltage. Accordingly, the FET 3 operates in the maximum ON time determined by the resistance value of the shunt resistor 51, irrespective of a speed at which the OFF control capacitor 53 is charged. The maximum ON time is determined such that energy stored in the transformer 2 becomes slightly greater than the sum of energy consumed by a load of rated power consumption and energy consumed by the switching operation of the self-excited switching power supply circuit 100. As a result, the output voltage increases to the set voltage while oscillating operation is repeated. If the output voltage exceeds the set voltage, a transition is made to continuous self-excited oscillating operation that is generally performed under the constant voltage output control.
During the OFF operation period of the FET 3, the output control capacitor 55 is also charged with the flyback voltage generated in the second feedback winding 2d through the rectifying diode 54. If the output voltage between the high-voltage side output line 20a and the low-voltage side output line 20b is higher than the set voltage when the FET 3 is turned on, the photocoupler light emitting element 35 emits light of an amount corresponding to a difference between the output voltage and the set voltage. Then, the photocoupler light receiving element 39 optically coupled to the photocoupler light emitting element 35 causes a discharging current proportionate to the difference between the voltages to flow from the output control capacitor 55 into the junction (base) J3 through the junction J2.
Thus, at a time immediately after the FET 3 is turned on, the OFF control capacitor 53 charged by voltage drop across the shunt resistor 51 caused by the exciting current flowing in the primary winding 2a is further charged with the charging voltage of the output control capacitor 55. This charges the OFF control capacitor 53 at a higher speed, so that the base voltage of the OFF control transistor 5 reaches the operating voltage within a time shorter than the maximum ON time.
This brings the gate of the FET 3 and the low-voltage side terminal 1b into a state where the gate of the FET 3 and the low-voltage side terminal 1b are substantially shorted with each other by the OFF control transistor 5, thereby turning the FET 3 off without delay after turn-on of the FET 3. Thus, the ON operation period is shortened in one oscillation cycle, and this reduces energy stored in the transformer 2 to lower the output voltage. The constant voltage control of the output voltage is realized by following the aforementioned processes.
The conventional self-excited switching power supply circuit 100 performs the constant voltage control in each oscillation cycle. The ON operation period in which the output voltage is changed under the constant voltage control is very short while the self-excited switching power supply circuit 100 performs continuous oscillating operation stably after the output voltage reaches the predetermined set voltage. Meanwhile, as long as the ON operation period is constant and power to be consumed by a load does not change, the OFF operation period is also constant in each oscillation cycle. Accordingly, the self-excited switching power supply circuit 100 operates at a substantially fixed oscillating frequency after the output voltage is stabilized at a level near the set voltage under the constant voltage control.
As an example, if the self-excited switching power supply circuit 100 is controlled under the constant voltage control to produce an output voltage to a load of 16 V and an output current of 1.2 A with a DC power supply voltage of 240 V, the ON and OFF operation periods in each oscillation cycle To do not change and stay at their substantially constant levels after the output voltage reaches the set voltage of 16 V. In this case, the self-excited switching power supply circuit 100 performs continuous oscillating operation with a fixed oscillation cycle of 160 kHz, generating harmonic noise between the output lines 20a and 20b caused by a second-order harmonic or a higher-order harmonic.
Regarding the characteristics of a noise voltage appearing in the output of a self-excited switching power supply circuit, International Standard CISPR22 for information technology equipment (ITE) specifies in class B upper limits determined in a QP mode (quasi-peak detection) and an AVR mode (average detection) in a frequency band of from 0.15 MHz to 30 MHz as shown in FIG. 7. If the self-excited switching power supply circuit 100 operates continuously with a fixed oscillation cycle of 160 kHz, second-order harmonic noise and harmonic noises of higher orders appear in the output of the self-excited switching power supply circuit 100 at frequencies of 320 kHz, 480 kHz, 640 kHz and others that are integral multiples of the fixed oscillation cycle. Margins with respect to the upper limits (1) and (2) specified in International Standard are reduced at these particular frequencies, generating a fear of excess of measured values (4) and (5) over the upper limits.
A noise component of a harmonic becomes greater with a lower order of an oscillating frequency. Accordingly, reduction of the oscillating frequency is desired. However, reduction of the oscillating frequency involves size increase of the self-excited switching power supply circuit 100. Further, provision of a large-capacity filter in a power supply line on the output side has been thought of, which results in increase of the cost of the entire circuit.